Add support for snap create-user on Ubuntu Core images.

Ubuntu Core images use the `snap create-user` to add users to an
Ubuntu Core system. Add support for creating snap users by adding
a key to the users dictionary.
  users:
    - name: bob
      snapuser: bob@bobcom.io

Or via the 'snappy' dictionary:
  snappy:
    email: bob@bobcom.io

Users may also create a snap user without contacting the SSO by
providing a 'system-user' assertion by importing them into snapd.

Additionally, Ubuntu Core systems have a read-only /etc/passwd such that
the normal useradd/groupadd commands do not function without an additional
flag, '--extrausers', which redirects the pwd to /var/lib/extrausers.

Move the system_is_snappy() check from cc_snappy module to util for
re-use and then update the Distro class to append '--extrausers' if
the system is Ubuntu Core.

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